(1) Field of the Invention
The present invention relates to an improved process for the preparation of polyphenylene-sulfide resins, in which low-molecular-weight by-products formed in preparing a polyphenylene-sulfide resin are effectively removed and a polyphenylene-sulfide resin characterized in that generation of a gas is controlled when the resin is mixed with a glass fiber or an inorganic filler and kneaded by an extruder and further in that a molded body having highly improved mechanical strengths is formed from the resin is obtained.
(2) Description of the Related Art
A polyphenylene-sulfide resin has heretofore been used mainly as a resin for injection molding, but this resin has a problem in that when the resin alone is used as a molding material, the resulting molded article has insufficient mechanical strengths, especially impact strength. Accordingly, the polyphenylene-sulfide resin is generally used in the state reinforced with a reinforcer such as a glass fiber or a carbon fiber. Furthermore, an inorganic filler such as talc or calcium carbonate is generally incorporated in the polyphenylene-sulfide resin. An extruder heated at about 300.degree. C. is ordinarily used for kneading the polyphenylene-sulfide resin with such a fibrous reinforcer or a filler. However, if kneading is carried out at such a high temperature, a gas having an extremely unpleasant smell is generated from a vent of the extruder, and therefore, the operation environment is often worsened. Even in the case of the polyphenylene-sulfide resin reinforced with the above-mentioned reinforcer, the impact strength is still not sufficient and an improvement is desired. Particularly, in the case of a thin molded article, an improvement of the impact strength is eagerly desired. The improvement of the mechanical strengths, especially the impact strength, of polyphenylene-sulfide is ordinarily accomplished by increasing the polymerization degree of polyphenylene-sulfide. To increase the polymerization degree of polyphenylene-sulfide, it is necessary that the reaction is carried out in the presence of an expensive polymerization assistant such as a lithium halide, an alkali metal carboxylate or an alkali metal sulfonate, as disclosed in Japanese Unexamined Patent Publication No. 59-115331. The polymerization assistant is used in such a large amount as 0.5 to 1.0 mole per mole of the aromatic polyhalide compound, and it is necessary to separate the polymerization assistant present in a large amount from the obtained polymer and perform the regeneration treatment. Therefore, this technique is not advantageous from the economical viewpoint.
Also a process for preparing polyphenylene-sulfide without using a polymerization assistant is known (see, for example, Japanese Examined Patent Publication No. 45-3,368). As the result of our investigation, it has been found that cyclic low-molecular-weight polymers and non-cyclic low-molecular-weight polymers are contained in the polyphenylene-sulfide resin obtained according to this process and if the content of the cyclic low-molecular-weight polymers is not controlled to a level lower than a specific value, generation of a gas cannot be controlled to a satisfactory extent and the mechanical strengths, especially the impact strength, cannot be substantially improved.